1. Technical Field
One or more embodiments of the present invention relate to capacitive sensors, and in particular, to a capacitive sensor of a microscopic size manufactured using an MEMS (Micro Electro Mechanical System) technique or a micromachining technique.
2. Background Art
(Manufacturing Method)
A capacitive sensor of a microscopic size is manufactured using an MEMS technique or a micromachining technique, as mentioned above. For example, the conventional capacitive vibration sensor (microphone) is manufactured through steps as shown in FIGS. 1A to 1F. The steps will be briefly described below.
First, as shown in FIG. 1A, surfaces of a Si substrate 11 are oxidized through a thermal oxidation method, and the surfaces of the Si substrate 11 is protected with a thermally oxidized film (SiO2 film) 12. Then, as shown in FIG. 1B, a vibration electrode plate 13 (movable electrode plate) is formed by a polysilicon film on the thermally oxidized film 12 at an upper surface of the Si substrate 11. As shown in FIG. 1C, a sacrifice layer 14 made of SiO2 is deposited on the upper surface of the Si substrate 11 from above the vibration electrode plate 13, and the sacrifice layer 14 is etched to form a mesa type sacrifice layer 14. Furthermore, a back plate 15 is formed by depositing SiN on the upper surface of the Si substrate 11 from above the sacrifice layer 14, and then a fixed electrode 16 made of metal thin film is formed on the back plate 15 to form a fixed electrode plate 17 including the back plate 15 and the fixed electrode 16. Subsequently, as shown in FIG. 1D, a plurality of acoustic perforations 18 is opened in the fixed electrode plate 17 by etching.
Thereafter, as shown in FIG. 1E, a window 19 is opened in that thermally oxidized film 12 on the back surface side, and the Si substrate 11 is anisotropically etched from the window 19 to form a hollow part 20. The hollow part 20 is reached up to the upper surface of the Si substrate 11 to pass the hollow part 20 through the Si substrate 11. Then, as shown in FIG. 1F, the sacrifice layer 14 is removed by etching through the hollow part 20 and the acoustic perforation 18, and a vibration electrode plate 13 that can vibrate is arranged in a space between the Si substrate 11 and the fixed electrode plate 17 to obtain a chip-shaped vibration sensor 23.
FIG. 2(a) is a schematic plan view of a vibration sensor 23 manufactured as mentioned above, and FIG. 2(b) is a plan view showing a state in which the vibration electrode plate 13 is exposed by removing the fixed electrode plate 17. The reference numeral 24 indicates an electrode pad electrically conducted with the fixed electrode 16 of the fixed electrode plate 17, and the reference numeral 25 indicates an electrode pad electrically conducted with the vibration electrode plate 13. The vibration electrode plate 13 has the four corner portions formed as a supporting leg 26 fixed to the Si substrate 11.
However, in the conventional vibration sensor 23, the strength near a fixed portion of the back plate 15, in particular, the strength of a side wall portion raised from the end of the fixed portion easily lowers because it is formed as shown in FIGS. 1A to 1F. FIGS. 3A and 3B are views for describing the reason the strength of the side wall portion lowers in the manufacturing steps of the vibration sensor.
FIG. 3A is a partially enlarged cross-sectional view of a state in which SiN is deposited on the mesa type sacrifice layer 14 to form the back plate 15, and shows the initial step of FIG. 1C. In the deposition step of SiN, the film growth rate becomes greatest in a vertical direction, and thus the film thickness is greatest in a horizontal plane of the back plate 15 and the film thickness becomes thinner than the horizontal plane at the side wall portion 21. Furthermore, the film quality is worse at the side wall portion 21 than the horizontal plane.
In the step of forming the hollow part 20 of FIG. 1E, the hollow part 20 is formed by performing wet etching on the Si substrate 11 with TMAH and KOH solution or performing dry etching using XeF2 gas. In this case, the back plate 15 is also simultaneously etched to some extent and particularly the thickness of the side wall portion 21 tends to be thin.
Furthermore, in the step of removing the sacrifice layer 14 by etching of FIG. 1F, the sacrifice layer 14 is removed by performing wet etching with hydrofluoric acid aqueous solution or performing dry etching using CF gas. In this case, the back plate 15 is also simultaneously etched to some extent and particularly the thickness of the side wall portion 21 tends to be thin.
Thus, as a result of etching the back plate 15 as indicated by arrows in FIG. 3B, the thickness of the side wall portion 21 becomes thin and the film quality thereof also degrades, whereby the strength of the side wall portion 21 of the back plate 15 becomes lower than other areas. Furthermore, as shown in FIG. 3B, a crack α tends to easily form at the boundary of the fixed portion 22 and the side wall portion 21 of the back plate 15 at the time of forming the back plate 15 and at the time of etching the Si substrate 11 and the sacrifice layer 14.
Thus, the stress has no place to escape when impact is externally applied to the vibration sensor 23, whereby the stress concentrates at the side wall portion 21 of the back plate 15 and the boundary of the side wall portion 21 and the fixed portion 22, thereby forming the crack in the back plate 15 and breaking the same.
The strength of the back plate 15 can be increased by increasing the film thickness at the time of forming the back plate 15, but the productivity of the vibration sensor 23 worsens because the film forming time becomes longer and the processing accuracy of the back plate lowers with such a countermeasure method, and thus it is not practical.
(Disclosure in Patent Document 1)
Patent Document 1 discloses a sensor in which the rigidity of the back plate (thin film plate of silicon nitride) is increased by forming the side wall portion of the back plate as a rib structure thereby preventing the warp of the back plate. In such a structure, the rib structure is formed at the side wall portion of the back plate, and thus the strength of the side wall portion appears to have increased.
However, as in the above structure, the side wall portion having the rib structure cannot prevent the etching of the back plate when etching the substrate and the sacrifice layer and cannot make the film quality of the side wall portion satisfactory when forming the back plate. Therefore, it is not effective in enhancing the strength of the side wall portion.
If the side wall portion has a rib structure, the stress rather tends to easily concentrate at the rib when impact is externally applied, thereby forming cracks at the side wall portion, the base portion thereof, and the like and breaking the same.
Patent Document 1: Japanese Unexamined Patent Publication No. 2007-116721 (FIG. 13, FIG. 14)